Despite decades of work in humans and animal models, the role of maternal/fetal microchimerism (the long term survival of rare cells from mothers in children or children in mothers) in reproductive immunology, pathogenesis of autoimmunity, and transplantation tolerance remains a largely unexplored area. A picture has begun to emerge, however-that maternal microchimerism somehow teaches the developing adaptive immune system how to balance the need for pathogen resistance against the requirement to protect sensitive tissues from autoimmune destruction. A key discovery in the course of the previous funding period was the finding in a mouse model of a link between the level of maternal microchimerism and the level of T regulatory cells specific for non-inherited maternal antigens or NIMA in adult mice. This discovery was paralleled by similar findings in healthy adult humans, suggesting general principles in mammals and not simply a 'mouse phenomenon'. Therefore we propose the hypothesis that maternal/fetal cell exchanges result in lifelong microchimerism that may evoke either tolerance or sensitization, depending on the ability of the chimerism to successfully infiltrate professional antigen-presenting cell [APC] lineages and thereby tip the balance in favor of T regulator vs. T effector cell induction. This hypothesis will be tested by means of 3 Specific Aims: 1)[Human] to determine in healthy families of different ethnic backgrounds the NIMA-specific immune status of individual family members, how this status is related to maternal microchimerism, and which alloantigen- specific T cells contribute to NIMA tolerance [T regulatory cell] or sensitization [T effector cell]; 2) [Mouse] to determine the role of particular Mc+ lineages in NIMA-specific tolerance or sensitization, and whether recipient only, or both recipient and donor (passenger) CD4 and CD8 T cells influence the NIMA effect on organ allograft outcome (the mutual regulation hypothesis); and 3) [Human] to determine if pre-transplant fetal/maternal immune status of recipient and/or donor predicts renal allograft outcome in HLA-haplo-identical living-related donor renal transplantation. Since the first submission of this competitive renewal application, we have discovered that mouse heart transplant tolerance to a NIMA-expressing donor can be accurately predicted prior to transplantation by simply measuring the level of indirect pathway anti- NIMA Treg activity [Dutta et al., AJT 2011, in press]. The significance of the research is that if successful, it will not only identify the key cellular components of natural allotolerance in mice and humans, but it will also enable transplant surgeons to select live-related patient-donor combinations with a high probability of successful outcome while avoiding sensitization in clinical tolerance trials.